Semiconductor wafer processing operations involve the performance of various types of processing steps or sequences upon a semiconductor wafer upon which a number of die (e.g., a large or very large number of die) reside. The geometrical dimensions, linewidths, or feature sizes of devices, circuits, or structures on each die are typically very small, for example, micron, submicron, or nanometer scale. Any given die includes a large number of integrated circuits or circuit structures that are fabricated, processed, and/or patterned on a layer-by-layer basis, for instance, by way of processing steps performed upon wafers sitting on planar wafer surfaces, such that the dies carried by the wafer are collectively subjected to the processing steps.
A wide variety of semiconductor device processing operations involve a number of handling systems that perform wafer or film frame handling operations which involve securely and selectively carrying (e.g., transporting, moving, displacing, or conveying) wafers or wafers mounted on film frames (hereafter referred to as “film frame” for brevity) from one position, location, or destination to another, and/or maintaining wafers or film frames in particular positions during wafer or film frame processing operations. For instance, prior to the initiation of an optical inspection process, a handling system must retrieve a wafer or a film frame from a wafer or film frame source such as a wafer cassette, and transfer the wafer or film frame to the wafer table. The wafer table must establish secure retention of the wafer or film frame to its surface prior to the initiation of the inspection process, and must release the wafer or film frame from its surface after the inspection process is complete. Once the inspection process is complete, a handling system must retrieve the wafer or film frame from the wafer table, and transfer the wafer or film frame to a next destination, such as a wafer or film frame cassette or another processing system.
Various types of wafer handling systems and film frame handling systems are known in the art. Such handling systems can include one or more mechanical or robotic arms configured for performing wafer handling operations which involve the transfer of wafers to and the retrieval of wafers from a wafer table; or performing film frame handling operations which involve the transfer of film frames to and the retrieval of film frames from a wafer table. Each robotic arm includes an associated end effector which is configured for retrieving, picking up, holding, transferring, and releasing a wafer or a film frame by way of the application and cessation of vacuum force relative to portions of the wafer or film frame, in a manner understood by one of ordinary skill in the relevant art.
A wafer table itself can be viewed or defined as a type of handling system, which must reliably, securely, and selectively position and hold a wafer or film frame on a wafer table surface while displacing the wafer or film frame relative to elements of a processing system, such as one or more light sources and one or more image capture devices corresponding to an optical inspection system. The structure of a wafer table can significantly impact whether an inspection system can achieve a high average inspection throughput, as further detailed below. Furthermore, the structure of a wafer table, in association with the physical characteristics wafers and the physical characteristics of film frames, greatly impacts the likelihood that an optical inspection process can reliably generate accurate inspection results.
With respect to the generation of accurate inspection results, during an optical inspection process, a wafer or a film frame must be securely retained upon the wafer table. Additionally, the wafer table must dispose and maintain the upper or top surface of the wafer or film frame in a common inspection plane, such that the surface areas of all wafer die, or as many wafer die as possible, collectively reside in this common plane, with minimum or negligible deviation therefrom. More particularly, the proper or accurate optical inspection of die at very high magnification requires a wafer table to be very flat, preferably with a planarity having a margin of error of less than ⅓ of the depth of focus of the image capture device. If the depth of focus of an image capture device is, for instance, 20 μm, a corresponding wafer table planarity error cannot exceed 6 μm.
For handling die of very small size (eg. 0.5×0.5 mm or smaller) and/or thickness (50 μm or less—e.g., carried by a very thin and/or flexible wafer or substrate), this planarity requirement becomes even more critical. For wafers that are very thin, it is important for the wafer table to be ultra-planar, otherwise it is easy for one or more die on the wafer or film frame to become positioned out of the depth of focus. One of ordinary skill in the art will recognize that the smaller the die, the higher the magnification required, and hence the narrower the band of depth of focus in which the inspection plane must lie.
With such planarity outlined aforesaid, a wafer placed on the wafer table will lie flatly on the wafer table surface, the wafer squeezing out substantially all the air beneath it. The difference in atmospheric pressure between the top and bottom surface of the wafer when the wafer is disposed upon the wafer table results in a large force applied against the top surface of the wafer due to atmospheric pressure, holding the wafer down strongly or reasonably strongly upon the wafer table. As pressure is a function of surface area, the larger the size of the wafer, the greater the force applied downwards on the wafer. This is commonly referred to as the “inherent suction force” or “natural suction force” on the wafer. The flatter the wafer table surface, the greater the natural suction force, up to the limit defined by the finite surface of the wafer. However, the strength of such suction force depends on how flat the wafer table surface is. Some wafer tables are not that flat and may have other grooves or holes on its surface resulting in reduced suction force. As the wafer table will be repeatedly accelerated over short distances during inspection of each die, and a high vacuum force is often applied through the wafer table to the wafer table surface to the underside of the wafer to ensure that the wafer remains as planar as possible and does not move during inspection; this is notwithstanding the presence of such natural suction force.
Various types of wafer table structures have been developed in attempts to securely hold wafers or film frames during wafer or film frame inspection operations, and reliably maintain a maximum number of die in a common plane during inspection operations. However not one design exists that will allow the wafer handling system to handle both wafers and sawn wafers mounted on film frames without one or more of the problems described below. A brief description will be made of each type of existing design and their associated problems.
Several types of wafer chucks have been or are currently in use. In the past, wafers were smaller (e.g., 4, 6, or 8 inches) and significantly thicker (particularly in relation to their overall surface areas, e.g., on a wafer thickness normalized to wafer surface area basis), and each die size was larger. Present-day wafer sizes are typically 12 or 16 inches, yet the thickness of these processed wafers have been decreasing in relation to their increasing size (for instance, thicknesses of 0.70-1.0 mm for 12-inch wafers prior to thinning/backgrinding/backlapping, and 50-150 μm following thinning/backlapping are common), and die sizes (e.g., 0.5-1.0 mm square), respectively. Standard wafer sizes can be expected to further increase over time. Additionally, thinner and thinner wafers can be expected to be processed each year in response to the increasing demands and requirements of electronics and mobile phone manufacturers for thinner die/thinner components to fit into slim-built electronic devices (e.g., flat screen televisions, mobile phones, notebook computers, tablet computers, etc.). As will be explained, these factors contribute to the increasing deficiencies of current designs of wafer table to handle both wafers and film frames.
Historically, and even presently, many wafer chucks have been made of a metal such as steel. Such metal wafer chucks are inlaid with a network of grooves, usually circular grooves that are intersected by grooves radiating linearly from a central location. Through such grooves, vacuum force can be applied to the underside of the wafer, which interfaces with the wafer table surface, in order to facilitate secure retention of the wafer against the wafer table surface. In many wafer table designs, such grooves are arranged in concentric circles of increasing size. Depending on the size of the wafer, one or more grooves would be covered by a wafer when the wafer is disposed upon the wafer table surface. Vacuum can be activated through the grooves covered by the wafer to hold the wafer down during processing operations, such as wafer inspection operations. After inspection, the vacuum is deactivated and ejector pins are deployed to lift the wafer off of the wafer table surface, such that the wafer can be retrieved or removed by an end effector. As there are linear grooves radiating from the centre of the metal wafer table surface, once the vacuum is deactivated, the residual suction force associated with application of the vacuum force to the underside of the wafer is quickly dissipated. Thicker wafers are more amenable to application of significant force applied through the ejector pins to lift the wafer (against any residual suction force, if any) without breaking.
As indicated above, increasingly wafers manufactured today are thinner or much thinner than before (e.g., present wafer thicknesses can be as thin as 50 μm), and each die thereon is also increasingly smaller in size (e.g., 0.5 mm square) than in the past. Technological progression results in smaller die sizes and thinner die, which pose a problem for handling wafers by way of existing wafer table designs. Very often, backlapped/thinned or sawn wafers (hereafter simply “sawn wafers”) having die that are very small in size and/or which are very thin are mounted on film frames for processing. Conventional metal wafer tables are not suitable for use with film frames having sawn wafers mounted thereto for a number of reasons.
Bearing in mind that inspection of die involves very high magnification, the higher the magnification, the narrower an acceptable depth of focus band, range, variance, or tolerance will be for accurate inspection. Die that are not in the same plane are likely to be out of the depth of focus of an image capture device. As indicated above, the depth of focus of a modern image capture device for wafer inspection typically ranges from 20-70 μm or smaller, depending on the magnification. The presence of grooves on the wafer table surface presents problems particularly during the inspection of sawn wafers mounted on film frames (with small die sizes) on such systems.
The presence of grooves results in the sawn wafers with small die sizes not sitting properly or uniformly on the wafer table surface. More particularly, in regions where there are grooves (and there can be many), the film frame's film can slightly sag into the grooves, resulting in the whole wafer surface lacking collective or common planarity across all die, which is critical for optical inspection operations. This lack of planarity becomes more pronounced for small or very small die of sawn wafers. Furthermore, the presence of a groove can cause die to be displaced at an angle relative to a common die inspection plane, or cause the die to sag and sit at one or more different and lower planes. Furthermore, light shining on tilted die which have sagged into grooves will reflect light away from the image capture device, such that the capture of an image corresponding to a tilted die will not contain or convey precise details and/or features of one or more regions of interest on the die. This will adversely affect the quality of images captured during inspection, which can lead to inaccurate inspection results.
Several prior approaches have attempted to address the aforementioned problems. For instance, in one approach a metal wafer table support includes a network of grooves. A flat metal plate is placed on top of the network of grooves. The metal plate includes many small or very small vacuum holes that allow vacuum to be applied through the perforations against a wafer or sawn wafer. Depending on the size of wafer under consideration, an appropriate pattern or number of corresponding grooves will be activated. While multiple small or very small vacuum holes can increase the likelihood that die can be collectively maintained in the same inspection plane, collective die planarity problems are still not effectively or completely eliminated due to continuing technological evolution that results in smaller and smaller die sizes and decreasing die thicknesses over time Such designs also include multiple sets of ejector pin triplets corresponding to different wafer sizes, i.e., multiple distinct sets of three ejector pins corresponding to multiple standard wafer sizes that the wafer table is capable of carrying. The presence of numerous holes for ejector pins can also present, and quite possibly worsen, collective die planarity problems when inspecting die carried on film frames, for reasons analogous to those set forth above.
Some manufacturers use wafer table conversion kits, in which a metal wafer table with grooves is used for handling whole wafers, and a metal wafer table cover with many very small openings is used for film frame handling. Unfortunately, conversion kits require inspection system downtime due to the fact that conversion from one type of wafer table to another, and post-conversion wafer table calibration, is time consuming and done manually. Such downtime adversely affects average system throughput (e.g., overall or average throughput with respect to both wafer and film frame inspection operations considered in sequence or together), and hence inspection systems that require wafer table conversion kits are undesirable.
Other wafer table designs, such as described in U.S. Pat. No. 6,513,796, involve a wafer table receptacle that allows for different central wafer table inserts depending on whether wafers or film frames are being processed. For wafer inspection, the insert is typically a metal plate with annular rings having vacuum holes for activation of vacuum. For film frames, the insert is a metal plate having many fine holes for vacuum activation, which can still give rise to collective die nonplanarity as described above.
Still other wafer table designs, such as disclosed in U.S. Patent Application Publication 2007/0063453, utilize a wafer table receptacle having a plate type insert consisting of a porous material in which distinct regions are defined by annular rings made of a thin film material. Typically, such wafer table designs are complex in construct and involves a delicate and complex manufacturing process, and hence difficult, time consuming, or costly to manufacture. Moreover, such designs can utilize metal annular rings to facilitate regional vacuum force control across the wafer table surface in accordance with wafer size. Metal annular rings can require undesirably long planarization times, or damage a polishing device that is used to polish the wafer table surface when planarizing the wafer table surface. Furthermore, metal rings can give rise to nonplanarity due to differential material polishing characteristics across the wafer table surface, and therefore metal annular rings are unsuitable for modern optical inspection processes (e.g., particularly involving sawn wafers mounted on film frames).
Unfortunately, prior wafer table designs are (a) unnecessarily structurally complex; (b) difficult, expensive, or time consuming to fabricate; and/or (c) unsuitable for various types of wafer processing operations (e.g., die inspection operations, particularly when die are carried by a film frame) as a result of insufficient wafer table surface planar uniformity in view of technological evolution that continues to give rise to smaller and smaller wafer die sizes and/or progressively decreasing wafer thicknesses. A need clearly exists for a wafer table structure and an associated wafer table manufacturing technique that that will enable the wafer table to handle both wafers and sawn wafers and which overcomes one or more of the foregoing problems or drawbacks.
In addition to the above aspects of wafer table design that can impact the accuracy of wafer and film frame inspection as well as average inspection throughput, multiple other types of wafer or film frame handling problems can exist, which can adversely affect wafer or film frame inspection operations. Such problems and prior art solutions thereto are detailed hereafter.
Wafer—Wafer Table Retention Failure Due to Wafer Non-Planarity
One type of wafer handling problem arises as a result of wafer non-planarity or warpage. This problem arises from a number of factors, including (a) the increasing size of wafers being manufactured; (b) the decreasing thickness of wafers being handled; and (c) the manner in which wafers are handled or stored prior to and after processing. Prior to and after processing such as optical inspection, wafers are held at their edges in a cassette. Given the increasing diameter and thinness of wafers, and the manner in which wafers are held in the cassette, sagging of a wafer near its center, or wafer warpage, is not uncommon. In addition, during backlapping processes to thin the wafer to required dimensions, the backlapping process can cause the wafer to have a reverse warp, although this problem is less common.
When a non-planar wafer rests upon a wafer table surface, a vacuum force applied through the wafer table surface which is intended to securely hold the entire bottom surface of the wafer against the wafer table surface will only weakly hold a portion of the bottom wafer surface. As the other portions of the wafer will reside above the wafer table surface and vacuum that is applied through the wafer table will leak and any residual vacuum force applied would be very weak. In such a situation, the wafer will not be held down securely and furthermore such a warped wafer 10 cannot, typically, be reliably inspected or tested.
Prior approaches directed to ensuring that the entire surface area of a wafer is securely held upon a wafer table surface involve automatically halting inspection system operation when an insufficient vacuum retention force (or vacuum leakage that is below a minimum vacuum retention threshold value) is detected, until an inspection system operator or user manually intervenes. To solve the problem, the inspection system operator manually presses the wafer against the wafer table surface, until vacuum force applied through the wafer table surface engages the wafer's entire surface area and securely retains the wafer against the wafer table surface. Such automatic halting of inspection system operation as a result of insufficient vacuum retention of the wafer upon the wafer table surface can only be resumed after user intervention to manually correct the problem. Such downtime adversely impacts system throughput.
Unpredictable/Uncontrollable Lateral Wafer Displacement Following Vacuum Force Cessation
Typically, for inspection of wafers, the following steps occur to place a wafer on a wafer table: (a) the wafer is retrieved from a cassette and sent to a wafer (pre)aligner; (b) the wafer aligner serves to properly orientate the wafer for inspection; (c) after wafer alignment is completed, an end effector conveys the wafer to a predetermined position where its center coincides with the center of the wafer table; (d) ejector pins are activated to receive the wafer; (e) the end effector lowers the wafer onto the ejector pins before retracting; and (f) the ejector pins then lower the wafer onto the wafer table for inspection while vacuum is applied to hold the wafer down for inspection.
When inspection is completed, (a) vacuum is deactivated; (b) the wafer is lifted up by the ejector pins; (c) the end effector slides beneath the wafer and lifts up the wafer; and (d) the end effector transfers the inspected wafer back to a cassette, and puts the wafer into the cassette.
It is pertinent to note that to enable the effector to place the wafer into the cassette, it is important that the wafer remains in a predetermined position, and has not changed its position, relative to the end effector from the time it was placed on the wafer table. This means that the wafer must not move from the moment it is placed on the table. If the wafer is significantly or seriously out of position relative to the end effector, there is a risk that the wafer can drop during conveyance, or be damaged when the end effector tries to push the off-positioned wafer into the cassette. To prevent these mishaps, when the wafer is finally picked up by the effector after inspection, the wafer should, relative to the end effector, be in the same position as it was when the wafer was placed on the wafer table prior to the start of inspection. To hold the wafer in its position upon placement by the end effector, vacuum through the grooves is activated in addition to the natural suction that results when the whole or parts of the wafer sits flatly on the wafer table.
In certain situations, after the application of vacuum force or negative pressure to the underside of a wafer has ceased, the wafer can slide laterally along the wafer table surface as a consequence of subsequent events or process steps. Unpredictable lateral motion of the wafer causes the wafer to move or translate to a different position from the position at which the wafer was originally placed upon the wafer table prior to or at the start of inspection (i.e., the wafer laterally slides away from a reference wafer table position relative to which the effector deposits and retrieves the wafer). Consequently, when the effector retrieves a wafer that is unreliably or unpredictably mispositioned as a result of such lateral motion, there is a risk that the wafer will be dropped or damaged when the effector attempts to load the out-of-position wafer 10 back into a wafer cassette.
Prior approaches for managing unintended lateral wafer displacement relative to a wafer table surface following vacuum force cessation involve manual intervention, which again results in the interruption of inspection or test system operations, adversely impacting production throughput.
Wafer—Film Frame Rotational Misalignment
At a particular stage of wafer manufacturing, wafers may be mounted on film frames. For instance, when wafers are to be sawn, they are usually mounted on film frames. After being sawn, the sawn wafers on film frame are further inspected for cosmetic and/or other types of defects. FIG. 1A is a schematic illustration of a wafer 10 mounted on a film frame 30, which carries the wafer 10 by way of a thin material layer or film 32 that typically includes an adhesive or tacky side to which the wafer 10 is mounted, in a manner readily understood by one of ordinary skill in the art. The wafer 10 includes a number of die 12, which are separated or delineated from each other by horizontal gridlines 6 and vertical gridlines 8 that are produced or which become evident during manufacture. Such horizontal and vertical gridlines 6, 8 correspond to or delineate horizontal and vertical sides 11, 16 of the die, respectively. One of ordinary skill in the art will understand that a wafer 10 typically includes at least one reference feature 11, for instance, a notch or a straight portion or “flat” segment on an otherwise circular periphery, to facilitate wafer alignment operations. One or ordinary skill in the relevant art will further understand that the film frame 30 includes a number of registration or alignment features 34a-b to facilitate film frame alignment operations. The film frame 30 can also include a number of other reference features, such as “flats” 35a-d. 
With respect to optical inspection, die 12 on the wafer 10 are automatically inspected or examined in accordance with inspection criteria that facilitate the identification of cosmetic or other (e.g., structural) defects on the die 12. Die 12 which meet the inspection criteria, as well as die which fail to meet the inspection criteria, can be tracked or categorized in accordance with “pass” or “fail” designations, respectively. Die 12 that successfully meet all inspection criteria are suitable for further processing or incorporation into an integrated circuit package, whereas die 12 that fail to meet all inspection criteria can be (a) discarded; (b) analyzed for determining failure cause(s) and preventing future failures; or (c) in certain situations, reworked/reprocessed.
Optical inspection involves directing illumination at individual die 12 or an array of die 12; capturing illumination reflected from the die 12 using an image capture device and generating image data corresponding to the die 12; and performing image processing operations upon the image data to determine whether one or more types of defects are present on the die 12. Optical inspection is typically performed “on-the-fly” while the wafer 12 is in motion, such that the die 12 carried by the wafer 12 are continuously moving relative to the image capture device during image capture operations.
Inspection of an entire wafer 10 requires the generation of an inspection result (e.g., a pass/fail result) corresponding to each die 12 on the wafer 10. Before an inspection result corresponding to any given die 12 can be generated, the entire surface area of the die 12 must first be completely captured. In other words, complete inspection of any given die 12 requires that the die's entire surface area must first be completely captured by the image capture device, and image data corresponding to the entire surface area of each of the die 12 must be generated and processed. If image data corresponding to the die's entire surface area has not been generated, image processing operations corresponding to the die 12 cannot be completed, and an inspection result cannot be generated, until the capture of a set of images encompassing the entire surface area of the die 12, or an “entire-die image,” has occurred. Therefore, if image data corresponding to the entire surface area of a die 12, or entire-die image data, has not been generated, the generation of an inspection result for the die 12 is unnecessarily delayed, which adversely affects inspection process throughput.
The greater the number of image capture operations required to completely capture the entire die image for image processing, the lower the throughput for inspection. It stands to reason that in order to maximize inspection process throughput, every die's entire surface area should preferably be captured in as few images as possible.
Error in the orientation of the wafer 10 can arise during the mounting of the wafer 10 on a film frame 30. In general, the error in wafer mounting relates to a wafer flat or notch 11 not aligning properly with respect to a given film frame reference feature, such as a film frame flat 35a. FIG. 1B is a schematic illustration of a wafer 10 that is rotationally misaligned relative to a film frame 30 that carries the wafer 10. It can be clearly seen that the wafer 10 shown in FIG. 1B bears a significantly different rotational orientation relative to its film frame 30 than the wafer 10 shown in FIG. 1A bears in relation to its film frame 30. More particularly, it can be seen from FIG. 1B that with respect to a horizontal reference axis 36 and/or a vertical axis 38 defined parallel to and perpendicular to a first film frame flat 35a, respectively, a pair of reference horizontal and vertical wafer gridlines 6, 8 are rotated, angularly offset, or misaligned by an angle θ compared to the wafer 10 shown in FIG. 1A.
In other words, for the wafer 10 shown in FIG. 1A, the angle θ, which indicates an angular extent to which a wafer gridline 6, 8 has been rotated away from a reference axis 36, 38 having a predetermined orientation relative to the first film frame flat 35a, is approximately zero. For the wafer 10 shown in FIG. 1B, the wafer-to-film frame misalignment angle is θ non-zero. As wafer size increases, and particularly for larger wafer sizes (e.g., 12 inches or greater), the rotational misalignment of a mounted wafer 10 vis-à-vis the film frame 30 typically creates problems during inspection of the wafer 10 mounted thereon, as further detailed hereafter.
During the capture of a given image of a die 12, an inspection system's image capture device can capture illumination reflected from only those portions of the die's surface area which are disposed within the image capture device field of view (FOV). Portions of the die's surface area which fall outside of the image capture device FOV cannot be captured as part of this image, and must be captured as part of another image. As indicated above, the maximization of inspection process throughput requires that the entire surface area of every die 12 on the wafer 10 be captured in as few images as possible. When multiple image capture operations are required to generate image data corresponding to a die's entire surface area, the generation of an inspection result for the die 12 is delayed, which adversely affects throughput. Each die 12 on the wafer 10 must therefore be properly aligned relative to the image capture device FOV in order minimize the number of image capture operations required to generate entire-die image data for all die 12 on the wafer 10, in order to maximize inspection process throughput.
Proper alignment of the die 12 relative to the image capture device FOV can be defined as a situation in which any rotational or angular misalignment of the die 12 relative to the image capture device FOV is sufficiently small, minimal, or negligible that the die's entire surface area will fall within the FOV. FIG. 2A is a schematic illustration of a die 12 that is properly positioned or aligned relative to an image capture device field of view (FOV) 50. As clearly indicated in FIG. 2A, under conditions of proper die alignment relative to the FOV 50, a horizontal border or side 14 of the die 12 is aligned substantially parallel to an FOV horizontal axis XI, and a vertical border or side 16 of the die 12 is aligned substantially parallel to an FOV vertical axis YI. Consequently, the entire surface area of such a die 12 falls within the FOV 50, and the entire surface area of the die 12 can be captured by the image capture device in a single image capture event, operation, or “snap.”
FIG. 2B is a schematic illustration of a die 12 that is improperly positioned or which is misaligned relative to an image capture device FOV 50. FIG. 2B clearly indicates that the horizontal and vertical sides of the die 14, 16 are rotated or angularly offset from the FOV horizontal axis XI and the FOV vertical axis YI, respectively, and portions of the die's surface area fall outside of the FOV 50. Because of such misalignment of the die 12 relative to the FOV 50, the generation of image data corresponding to the entire surface area of the die 12 requires the capture of multiple images that capture different portions of the die 12, resulting in reduced inspection process throughput. More particularly, as shown in FIG. 2C, up to four images may be required to capture the entire surface area of such a rotationally misaligned die 12, depending upon the extent of the die's misalignment relative to the FOV.
When film frames are handled, typically a mechanical film frame registration procedure must take place. Usually, the film frame registration procedure occurs when the film frame is placed on the wafer table. In some systems, such as that described in Singapore Patent Application No. 201103524-3, entitled “System and Method for Handling and Aligning Component Panes such as Wafers and Film Frames,” filed on 12 May 2011, a mechanical film frame registration can take place before placement of the film frame on the wafer table, such as when an end effector that carries the film frame causes a set of film frame alignment features 34a-b to engage with film frame registration elements or structures prior to placement of the film frame on the wafer table.
A mechanical film frame registration procedure involves a certain amount of handling time. However, the film frame registration procedure typically ensures that the film frame 30 is properly aligned or registered with respect to the image capture device FOV. However this assumes that the wafer was properly mounted on the film frame in the first place, which is not always the case. Where the wafer mounted on the film frame has a rotational misalignment, it can give rise to problems and delays in inspection, adversely affecting throughput as elaborated upon below.
The film frame registration procedure occurs by way of mating engagement between film frame registration features 34a-b and one or more film frame registration elements, which are conventionally carried by a wafer table assembly. After a film frame 30 has been registered, die 12 on the wafer 10 mounted to the film frame 30 are expected to be properly aligned with respect to the image capture device FOV. However, if more than a slight or minimal amount of rotational or angular misorientation of the wafer 10 mounted to the film frame 30 exists, the die 12 will not be properly aligned relative to the image capture device FOV. It therefore stands to reason that the extent of any rotational misalignment of a wafer 10 that occurs during the mounting of the wafer 10 to a film frame 30 can adversely affect the number of images required to capture the entire surface area of each die 12 on the wafer 12, and hence the extent of any rotational misalignment of the wafer 10 relative to the film frame 30 can adversely affect inspection throughput.
Proper alignment of the wafer 10 relative to its film frame 30 ensures proper alignment of the die 12 relative to the image capture device FOV 50. Proper alignment of the wafer 10 relative to its film frame 30 can be defined as a situation in which one or more wafer gridlines 6, 8 have a standard predetermined alignment relative to one or more film frame structural features such as film frame flats 35a-d and/or the image capture device FOV, such that the each die 12 is positioned relative to the image capture device FOV in the manner shown in FIG. 2A (i.e., each die's horizontal and vertical sides 14, 16, with the FOV horizontal and vertical axes XI and YI). Such alignment of the wafer 10 relative to the film frame 30 minimizes the number of image capture operations required to capture each die's entire surface area, thereby maximizing inspection process throughput.
To further illustrate, FIG. 2D is a schematic illustration of a wafer 10 that is properly mounted on and aligned relative to a film frame 30, and an inspection process wafer travel path along which an image capture device captures an image of the entire surface area of each die 12 within successive rows of die 12 on the wafer 10. Two representative rows of die 12 are identified in FIG. 2D, namely, row “A” die 12 and row “B” die. Because this wafer 10 is properly aligned relative to its film frame 30, during the inspection process the entire surface area of each die 12 within row “A” can be captured in a single corresponding image (e.g., while the wafer 10 is in motion, or “on-the-fly”). Following the capture of the images corresponding to the row “A” die, the wafer 10 is immediately positioned such that the surface area of a row “B” die 12 that is closest to the last considered row “A” die 12 can be captured by the image capture device, and inspection continues along an opposite direction of travel. Thus, the inspection travel path is “serpentine.” Once again, because this wafer 10 is properly aligned with respect to its film frame 30, during the inspection process the entire surface area of each die 12 within row “B” can be captured in a single corresponding image. Inspection of the entire wafer 10 in this manner, when the wafer 10 is properly aligned relative to its film frame 30, results in maximum inspection process throughput.
FIG. 2E is a schematic illustration of a wafer 10 that is rotationally misaligned relative to a film frame 30 that carries the wafer, and an inspection process wafer travel path along which an image capture device captures less than the entire surface area of each die 12 within successive rows of die 12 on the wafer 10 during any single image capture event. During an optical inspection process, as a result of such wafer-to-film frame rotational misalignment, the horizontal and vertical sides 14, 16 of the die 12 carried by the wafer 10 will be rotationally offset from the FOV horizontal and vertical axes XI and YI, respectively, even when the film frame 30 itself is properly registered with respect to the image capture device. Consequently, the entire surface area of a given die 12 may not fall within the image capture device FOV 50, and multiple individual images will be required to capture a given die's entire surface area. Because an inspection result cannot be generated for the die 12 until after multiple images have captured the die's entire surface area, the generation of an inspection result corresponding to the die 12 is undesirably delayed.
Analogous considerations to those described above apply when inspection involves a group of die 12. FIG. 2F is a schematic illustration of a die array 18 in which the collective surface area of all die 12 within the die array 18 is smaller than an image capture device FOV 50, and the die array 18 is properly aligned relative to the image capture device FOV 50 because the horizontal and vertical sides 14, 16 of each die 12 within the die array 18 are substantially parallel to the FOV horizontal axis XI and the FOV vertical axis YI, respectively. As a result, the entire die array 18 can be captured as a single image by the image capture device, thereby maximizing inspection process throughput. FIG. 2G is a schematic illustration of a die array 18 for which the horizontal and vertical sides 14, 16 of the die 12 within the die array 18 are not properly aligned with respect to the FOV horizontal and vertical axes XI and YI. Thus, portions of the die array 18 fall outside of the FOV 50. As a result, multiple images of the die array 18 must be captured before an inspection result can be generated for the die array 18, thereby lowering throughput.
Moreover, analogous considerations to those described above also apply when inspection involves a single (e.g., large) die 12 that, when properly aligned relative to the image capture device FOV 50, has a surface area that is larger than the FOV 50. FIG. 2H is a schematic illustration of a die 12 having a surface area that is larger than the FOV 50 of an image capture device. This die 12 is also properly aligned relative to the FOV 50, because the die's horizontal and vertical sides 14, 16 are substantially parallel to the FOV horizontal and vertical axes XI and YI, respectively. As a result, the overall surface area of the die 12 can be captured in a minimum number of image capture operations. In this example, the image capture device must capture a total of 9 images for inspection of the entire surface area of the die 12, which occurs by way of successively positioning different portions of the die's surface area relative to the image capture device, and capturing an image of each portion of the die's surface area that falls within the image capture device FOV 50 during each such relative positioning.
FIG. 2I is a schematic illustration of a single die 12 such as that shown in FIG. 2H, which under proper FOV alignment conditions would be completely inspected through the capture of 9 images, but for which horizontal and vertical die side misalignment relative to FOV horizontal and vertical axes XI and YI results in portions of the die 12 remaining outside of the image capture device FOV 50 even after 9 images have been captured.
Prior systems and methods rely upon either manual intervention or a rotatable wafer table to compensate or correct for rotational misalignment between a wafer 10 and a film frame 30. As before, manual intervention adversely affects system throughput. With respect to a rotatable wafer table, such a wafer table is configured for selectively providing an amount of rotational displacement that is sufficient to compensate or substantially compensate for wafer—film frame rotational misalignment. The magnitude of misalignment between a wafer 10 and a film frame 30 can span a significant number of degrees, for instance, 10-15 degrees or more, in a positive or negative direction. Unfortunately, a wafer table configured for providing such rotation is undesirably complex from a mechanical standpoint, and correspondingly expensive (e.g., prohibitively expensive). Furthermore, the additional structural complexity of a wafer table assembly that provides such rotational wafer table displacement can make it significantly more difficult to consistently maintain the wafer table surface in a single plane perpendicular to the optical axis of the image capture device during inspection.
A need exists for a wafer and film frame handling system that provides a single wafer table structure for handling both wafers and film frames, and which can automatically overcome at least some of the aforementioned problems arising from wafer warpage, unpredictable lateral wafer motion, and wafer—film frame rotational misalignment, and which can enhance or maximize inspection process throughput.